Conventional optical photographic and video cameras, telescopes and microscopes are used to display and/or record images. Such systems rely on reflective and refractive optical lenses. The refractive optical lenses serve to focus light within the systems. Typically, the lenses are made of glass or plastic, and exhibit fundamental characteristics substantially unchanged since the time of Galileo. Refractive optical lenses range in size from microscopic dimensions to meters across.
A refractive lens focuses an image by directing to a particular point on a focal plane photons originating at a corresponding point in an image plane. For purposes of discussion, the general lens problem may be simplified to the problem of a lens focused “at infinity.”
Generally, incoherent light diverges from a light source. At large distances from the light source, however, this divergence becomes negligible. Consequently, light arriving at a receiving device from a source at a large distance from the receiving device arrives along substantially parallel rays. The distance from the image plane at which light rays appear substantially parallel depends on the characteristics of the system sensing the light. For a typical photographic camera focusing beyond approximately 40 feet is equivalent to focusing at infinity.
Unfocused light uniformly illuminates a plane disposed in the path of the arriving light rays. This uniform illumination carries less information content than a focused image, in which variation of light intensity across the focal plane corresponds to variation of light intensity at the image plane.
One simple apparatus for forming a focused image is a pinhole aperture disposed in a substantially opaque barrier where the opaque barrier is disposed in spaced relation to a reflective or translucent screen. A pinhole camera includes an opaque barrier having an aperture therein. The pinhole camera provides a focused image on a reflective, translucent or optically sensitive screen that is disposed in spaced relation to the barrier. The focused image is related to the distribution of light arriving from a distant image plane.
The barrier blocks all of the light arriving at the barrier from a particular light source except for the portion of that light arriving incident to the aperture. Light arriving at the aperture passes through the aperture and impinges on the screen. Light arriving from different light sources arrives at different solid angles with respect to the barrier, and accordingly illuminates correspondingly different regions of the screen.
A pinhole camera uses available light inefficiently. The image includes only light arriving directly at the aperture. Other light arriving at the barrier is absorbed by, or reflected from, the barrier and is thus unavailable for image formation. Furthermore, the resolution of the image on the screen is limited by aperture size. A small aperture forms a higher-resolution image than a large aperture. A smaller aperture, however, allows a smaller proportion of the light arriving from a particular source to pass through to the screen, while a correspondingly larger portion of the incident light is reflected or absorbed by the barrier.
A refractive lens uses incident light much more efficiently to form an image. Typically, light arriving along parallel rays from a distant source is collected across an entire surface of a refractive lens. Wherever the light impinges on the lens, it is redirected towards a point on a focal plane. In an ideal case, image resolution on the order of the wavelength of the incident light can be achieved, and the efficiency of the system is high, since most of the light incident on the surface of the lens is transferred to the focal plane, rather than being reflected or absorbed.
While refractive lens systems provide relatively high efficiency and resolution, they have significant disadvantages. The geometry of a refractive lens is constrained by the index of refraction of the material or materials of which the lens is formed, and by the refraction effects desired. Consequently, the shape and volume of a refractive lens system is constrained within certain parameters. In particular, the depth of the lens system may be non-negligible in the overall design of an optical system. To some extent lens system thickness may be reduced by applying fresnel lenses, however use of a fresnel system implies other design constraints. In addition, optical materials having desirable refractive characteristics may be relatively dense, resulting in correspondingly heavy focusing systems.
Recent years have witnessed significant advances in electronic imaging technology. In particular, the technology of charge coupled devices and CMOS photosensors has developed rapidly. CMOS devices are now available with significant integrated processing capability such that photosensor arrays and digital signal processing devices are mutually disposed on a common substrate. Consequently, electronic photosensors are now employed in a wide variety of imaging applications and apparatus.
With digital electronic photosensors has come improved methods of image storage. Images acquired by digital cameras are readily adapted to be stored and manipulated in digital format. Such manipulation includes postprocessing of images acquired by conventional image acquisition systems to extract information present, but not readily visible, in the original image. Various algorithms and mathematical transform techniques have been applied to the processing of images acquired through refractive lens systems. Nevertheless there remains a need for compact and light-weight image acquisition systems capable of acquiring images with reduced mechanical complexity. In view of these and other limitations, there exists an opportunity to advance the state of the art.